1. Technical Field
This invention relates to the field of signaling systems for a telecommunications network and, more particularly, to a method and apparatus for reverting to in-band signaling in the event of the detection of a failure of presently conventional out-of-band signaling systems.
2. Description of the Relevant Art
In the United States, it became prevalent in the telecommunications arts to provide in-band signaling between switching centers through the middle of the twentieth century. Referring to FIG. 1, there is shown a simplified block diagram of the telecommunications network in the United States circa 1970. FIG. 1 is used by way of example to describe in-band signaling; it is not intended to comprise a thorough study of telecommunications networks, and the reader is referred to well-known telecommunications textbooks for further insight. While Local Exchange Carrier (LEC) was a term not known in the art in the middle of this century, LEC 180 signifies one of the several Bell telephone operating companies or independent telephone companies serving a local geographic area. Of course, in the middle of this century, there was only one toll carrier, American Telephone and Telegraph Company (AT&T), signified as toll carrier 190. At a time after the middle of this century, competition in toll services was permitted and other toll carriers entered the market. Other toll carriers would be shown as separate network clouds 190. Of course, there exists several LEC's 180, and each LEC connects to the several toll carriers, the LEC's providing service in their respective geographic areas.
A local subscriber 100 in a local exchange area is connected, according to the system of FIG. 1, by typically a two wire facility 110 to an end office 130. A second end office 135 is shown within the local area 180 connected to end office 130 by trunk/trunk group 140 for completion of local calls. The two wire facility 110 connecting a subscriber to an end office 130 is typically a copper wire pair and the end office 130, 135, typically, a No. 5 cross-bar switch manufactured by, then, Western Electric Company, and still supported today by Lucent Technologies, Inc. Generally, these crossbar type common-controlled switches of end offices 130, 135 and even older vintage switches such as step-by-step switches have been replaced for the most part by electronic switching systems. Typically, two or four wire trunk facilities 140 connect end office 130 of LEC 180 with other end offices, such as end office 135, in the local exchange carrier's network 180. These may be N carrier, T carrier, copper wire or other facilities then prevalent. A two wire or four wire facility 150 (a four wire facility is shown) is typically used to connect an end office 130 to an originating toll switch (OTS) 170. An originating toll switch 170, in turn, is typically connected to other toll switches of a toll network 190 by four wire facilities. By four wire facility is intended the use of tip and ring leads for each direction of transmission or simulated four wire facilities such as carrier facilities. Trunks 150 may be one way (such as originating or terminating trunks or trunk groups) or two-way (both originating and terminating at end office 130; two-way trunks may be seized and busied out by either end office 130 or originating toll switch (OTS) 170). Long distance trunks 155 are shown leaving originating toll switch 170 for connection to other toll switches (not shown) in toll network cloud 190. In the 1970's, analog L carrier facilities were typically utilized for long distance trunks 155, either via land line or microwave. Since then, these have been substantially replaced by digital trunk facilities (T1, T3, . . . hierarchy) in time division multiplex arrangement.
A customer 100 in the United States typically dials a toll/directory number (DN) given by a three digit area code and a seven digit address of a called party within the dialed area code to reach a called party outside the calling party's area code. (In some states such as Maryland, one must dial the area code and telephone number to make a local call.) The signaling data is provided to end office 130 via dial pulse or tone signals originating from a rotary dial switch or a tone signaling pad of a telecommunications terminal respectively of calling party 100. These are carried via two wire facility 110 to end office 130. In a toll call or a local call involving more than one office, the end office 130, 135 may repeat the so-called dial pulse (DP) address signals or generate different tone address signals as appropriate. In the latter case, for example, there is a translation of "touch-tone" to dual tone multifrequency (MF) signals at end office 130, 135 for transmission on trunk 150, 140.
Thus, for each trunk between switching centers, such as end office 130 and originating toll switch 170, there is a respective supervisory signaling and a DP or MF address signaling capability directly associated therewith. This direct association of a signaling path for supervisory and address message signaling with a voice/data trunk 150 is what is intended herein by in-band signaling. In-band signaling was prevalent and out-of-band signaling unknown in the telecommunication arts until the 1970's.
Referring to FIG. 2, and by way of example, there is shown a typical in-band signaling arrangement for T1 carrier. T carrier, developed in the late 1960's, has replaced analog carrier systems such as N and L carrier over time. T1 carrier utilizes a digital pulse code modulation scheme and permits the transmission of 24 voice/data channels over a 1.544 megabit per second data stream. A T1 carrier system comprises 24 individual voice/data channels. In a simple example, each channel may comprise one trunk connecting two end points.
Associated with each channel, according to prior art T1 systems, there would be, for example, the digital equivalent of an E and M (Ear and Mouth) lead or other supervisory signaling and an MF address signaling capability. Supervisory signaling comes in several varieties such as ground start and wink start depending, for example, on how and which office initiates the signaling. Each channel provides a digital bitstream for a voice or data channel and an in-band signaling data and a maintenance data path as will be further explained below.
Signaling for supervisory and address messages requires little bandwidth while voice/data consumes much greater bandwidth of a channel. Consequently, to utilize an entire voice/data channel for supervisory and address signaling came to be recognized as a waste of the voice/data bandwidth of the channel. There was recognized a need in the telecommunications art to preserve the bandwidth requirements. Also, the end-to-end links from a calling party 100 to a called party might involve tying up trunks connecting as many as nine switching centers of LEC 180, toll carrier 190 and a terminating LEC (not shown) according to the prior art. For example, a voice/data bandwidth signaling path would be required from End Office 130 of FIG. 1 all the way through a hierarchy of switching centers in a local/toll network configuration to reach a terminating end office (not shown). The waste of bandwidth per channel, thus, was further compounded by the unnecessary reservation of trunks and service circuits all across the country when it might be learned from the terminating end office that the called party would be "busy" or a switching center in the path would be otherwise unable to provide a link through the office, a service circuit or a trunk to the next office. Finally, the problem with in band signaling was further complicated by a problem with black box long distance line fraud. The black box could be used by a service pirate to steal telephone service by emulating the tones of MF signaling. Once a long distance line was seized via a toll-free call, a terminating toll office could be fooled to connect a caller to another number. With a switch to out-of-band digital packet signaling, the black box toll fraud was eliminated.
Also, while the in-band supervisory and address signaling capability remained with the T1 carrier channel, it was no longer utilized for its original purpose with the switch to out-of-band signaling. The T1 carrier channel unit, for example, typically may have unutilized MF wink start, address/supervisory signaling capability and bandwidth.
Out-of band signaling evolved, then, from the recognition of this waste of bandwidth, trunk resources and the desire to eliminate black box fraud. The first out-of-band signaling was known as Common Channel Interoffice Signaling (CCIS). CCIS provided, initially, a way of moving supervisory and address signaling, for example, from each T1 carrier channel to a single channel where the signaling information for a plurality of voice/data channels could be multiplexed together, saving having to tie up the plurality of voice/data capacity for signaling.
It also became recognized that such signaling could be provided end-to-end, from the calling party to the called party. It became an objective to build signal transfer points connected, for example, in a data network whereby a caller and a called party's identity could be shared between end points of a communication path. Once an originating end office, such as end office 130, signaled a terminating end office where a called party was located, then and not until then, the two offices are connected by a voice/data channel arranged through intermediary offices. The signaling functions is segregated from the voice/data transmission function of the network and is performed sequentially. The voice/data connection need not be established if the called party is busy.
The first out-of-band signaling protocol was known as CCS6. CCS6 has been replaced for the most part by CCS7 or common channel signaling system 7. An address signaling message packet is generated at an originating office. The packet carries, among other information, the address (data representing the phone number including area code) of the calling party 100 and the address (phone number) of the called party so that, once it is determined that the called party is not busy, a voice/data facility may be connected between the parties. From the toll network's perspective, the most important information of the called party address is the area code and first three digits of the seven digit telephone number signifying the terminating end office location. (Of course, this is a simplified explanation of simple voice services--alternative advanced services such as cellular, paging, call forwarding and voice messaging services are more complicated to provide.) With this information, a signaling data packet entering a signaling network can signal any signal transfer point reading the packet whether the token is destined for that signal transfer point.
Referring briefly to FIG. 3, each processor or auxiliary processor such as 3B processor 325 has links to a pair of signaling transfer points (STPs). These are shown as links connecting link nodes (LN's) on the ring 350. Simply stated, each signaling message contains a destination point code (address) where the message is destined for and an originating point code identifying where the message was sent from. STP's are duplexed (paired) to provide redundancy back-up, and, as will be further explained below, any failure of a single STP or connection will not cause a loss of service.
A problem has arisen in recent years when two signal transfer points of an out-of-band signaling system 350, such as SS7, may in rare instances break down. Such a calamity has occurred and may occur again. The entire country or a whole region of the country may not be able to originate or receive toll communications services due to such a breakdown. Other potential failures are a breakdown in an Application Peripheral Interface (API) or an auxiliary 3B processor at a #4ESS toll switch along with an Input/Output interface frame or F-link breakdown to a helper toll switch as will be further described herein.
One solution to the problem of an out-of-band signaling breakdown has been to provide redundancy in the signaling network. For example, each originating toll switch 300 may be connected to, not just one, but two signal transfer points. The two STP's form an STP pair. The second signal transfer point also provides access to the signaling network 350. Even with this redundancy, the possibility of a failure exists.
Referring briefly to FIG. 3, the redundancy solution, for example, may include providing access to a helper toll switch via an I/O frame and F links to gain access to another toll switch called a helper switch so the signaling packet may be sent to another signal transfer point (STP). Nevertheless, there may still be a severe signaling network breakdown. The loss of signal may not be due to the loss of an STP but a break in the ring, a loss of an auxiliary processor or other breakdown. Consequently, there is a need in the art for a method and apparatus for providing recovery from such a loss of out-of-band signaling capability.